Multicellular organisms are created from a complex organization of cooperating cells.
Some cells provide protection; some give structural support or assist in locomotion; others offer a means of transporting nutrients.
All cells develop and function as part of the organized system -- the organism -- they make up. There must be new mechanisms for cell to cell communication and regulation.
In humans, there are 10 14 cells comprising 200 kinds of tissues!
Each of us originated as a single, simple-looking cell -- a fertilized egg, or zygote -- so tiny that it can barely be seen without a microscope. ( A human egg cell is about 1/100th of a centimetre in diameter, or a bit smaller than the width of a human hair .)
Shortly after fertilization, the zygote begins dividing, replicating itself again and again. Before long, a growing mass, or blastula, of dozens, then hundreds, then thousands of cells called stem cells forms; each stem cell is only one-fourth to one-tenth the diameter of the original zygote, but otherwise nearly identical to it
Every nucleus of every cell has the same set of genes. A heart cell nucleus contains skin cell genes, as well as the genes that instruct stomach cells how to absorb nutrients.
Therefore, for cells to differentiate, certain genes must somehow be activated, while others remain inactive.
Genes instruct each cell how and when to build the proteins that allow it to create the structures, and ultimately perform the functions, specific to its type of cell.
Gene Regulation in Bacteria
Bacteria adapt to changes in their surroundings by using regulatory proteins to turn groups of genes on and off in response to various environmental signals
The DNA of Escherichia coli is sufficient to encode about 4000 proteins, but only a fraction of these are made at any one time. E. coli regulates the expression of many of its genes according to the food sources that are available to it.
The lac operon
The best understood cell system for explaining control through genetic induction is the lac operon
Jacob & Monod (1961) - regulation of lactose
metabolism in E.coli
Composed of 3 segments, or loci of DNA:
1. The REGULATOR - composed of gene that codes for a repressor protein which can repress the operon.
2. The CONTROL locus - consists of a promotor and the operator - can start transcription of the structural genes
3. The STRUCTURAL locus - contains structural genes encoding the enzyme -galactosidase
The lac operon In an E. Coli cell growing in the absence of lactose , a repressor protein binds to the operator, preventing RNA polymerase from transcribing the lac operon's genes. The operon is OFF When the inducer, lactose , is added, it binds to the repressor and changes the repressor's shape so as to eliminate binding to the operator. As long as the operator remains free of the repressor, RNA polymerase that recognizes the promoter can transcribe the operon's structural genes into mRNA. The operon is ON